With the growth of optical communication systems there is a continuing demand for lower-cost, high-performance optical components with extended flexibility in operation. Component integration is a key technology that provides the benefits of lower costs and reduced sizes as compared to solutions based on discrete components.
The dynamic range of an optical receiver is limited at low optical input powers by the coupling loss and photo-detector sensitivity. At high optical input powers the dynamic range is limited by the overload parameters that reflect the saturation level of the associated electronics, such as a transimpedance amplifier (TIA). In practical network applications a variable optical attenuator (VOA) is placed before the receiver and is used to control the received optical power within a limited range, assuring optimum receiver performance. The VOA functionality can also be used for receiver overload protection when, due to the system overload, the input optical power levels exceed the receiver damage threshold.
From the above perspective, integration of variable attenuation and receiver functions makes perfect sense, providing convenient closed-loop attenuation control based on receiver feedback for instant overload protection or performance optimization. Existing High Dynamic Range Integrated (HDRI) receivers, such as the ones disclosed by Y. Soskind et.al., “High Dynamic Range Integrated 10 Gb/s Receiver”, Proc. SPIE Photonics Packaging and Integration VI, Vol. 5358, pp. 20-28, 2004, provide integration benefits of improved performance, such as reduction in the total insertion loss, response time and optical components count.
Different types of optical attenuation mechanisms have been developed for VOA applications, including various types of Micro-Electro-Mechanical Systems (MEMS) structures, such as those disclosed in U.S. Pat. No. 6,782,185, entitled “Optical Variable Attenuator and Optical Module”, issued Aug. 24, 2004; U.S. Pat. No. 6,754,431 entitled “Variable Optical Attenuator”, issued Jun. 22, 2004; and U.S. Pat. No. 6,636,683 “Variable Optical Attenuator”, issued Oct. 21, 2003, liquid crystal devices such as those disclosed in U.S. Pat. No. 6,781,736, entitled “Folded Liquid-Crystal Variable Optical Attenuator”, issued Aug. 24, 2004; and U.S. Patent Applications Nos. 20040174473, entitled “Liquid crystal variable optical attenuator”, published Sep. 9, 2004; and 20040141710, entitled “Variable Optical Attenuator”, published Jul. 22, 2004; and waveguide structures such as those disclosed in U.S. Pat. No. 6,611,649, entitled “Variable Optical Attenuator with Polarization Maintaining Fiber”, issued Aug. 26, 2003; U.S. Pat. No. 6,493,478, entitled “Photothermal Optical Switch and Variable Attenuator”, issued Dec. 10, 2002; and U.S. Pat. No. 6,317,233, entitled “Optical Power Equalizer in WDM Optical Communication System and Variable Attenuator for Use Therein”, issued Nov. 13, 2001.
MEMS actuation mechanisms constitute a group of reliable cost-effective components well suited for high volume fabrication and packaging. VOA schemes with MEMS actuators may employ beam blockers, such as those disclosed in U.S. Pat. No. 5,909,078, entitled “Thermal Arched Beam Microelectromechanical Actuators”, issued Jun. 1, 1999, tilting mirrors, such as those disclosed in U.S. Pat. No. 6,754,431, entitled “Variable Optical Attenuator”, issued Jun. 22, 2004; and U.S. Pat. No. 5,915,063, entitled “Variable Optical Attenuator”, issued Jun. 22, 1999; and reflective diffractive structures, such as those disclosed on http://www.lightconnect.com/products/voa.shtml.
While designing an integrated product, receiver and VOA packaging considerations are equally important. Receiver packages performing O-to-E conversion typically employ optical and RF ports that oppose each other, See, for example, W. K. Hogan et. al., “Low-Cost Optical Sub-Assemblies for Metro Access Applications”, Proc. 54th Electronic Components and Technology Conference, paper s05p4, pp. 203-207, 2004, making in-line VOA optical layouts well suited for receiver integration. Optical blockers are well suited for in-line VOA layout, as disclosed in U.S. Patent Application No. 20030223727, entitled “Optical receiver with high dynamic range”, published Dec. 4, 2003 and assigned to JDS Uniphase Corporation, leading to small size implementation of HDRI receivers. FIG. 1 presents an optical layout of a conventional HDRI receiver with a beam blocking actuator. The output from the angle-polished input fiber 101 propagates through a ball lens 102 and is coupled to the active area 104 of a back-illuminated photo-detector 103. Thermally actuated beam blocker 105 is located in a divergent beam at a distance D from the input fiber 101. Lateral movement of the beam blocker 105 into the beam provides required attenuation, extending the dynamic range of the receiver.
Folded optical configurations are commonly employed to reduce the package size of discretely packaged VOAs using reflective MEMS devices. FIG. 2 presents a schematic optical layout of a conventional VOA employing a reflective actuator. The output from the input fiber 201 propagates through a collimating lens 202, is reflected by a VOA actuator 203 through the lens 202, and is coupled into an output fiber 204. When a reflective mirror is used in place of the VOA actuator 203, attenuation is achieved by changing the angular orientation of the mirror. When a diffractive structure is used in place of the VOA actuator 203, attenuation is achieved by adjusting the phase difference between interfering portions of the beam. To reduce the packaging cost and complexity of a VOA employing reflective actuators, both the input fiber 201 and the output fiber 204 are located on the same side of the VOA package, and are commonly sharing the package feed-through.
In some HDRI receiver applications; however, it is desirable to use reflective MEMS structures. Reflective electrostatic MEMS mirrors or diffractive structures require significantly lower actuation power as compared to that for thermally actuated beam blockers, and may be used when the HDRI power consumption is limited. Diffractive MEMS structures may also be used when attenuation response time of several tens of microseconds or less is required.
Integration of a reflective MEMS actuator into a receiver package requires the addition of optical components leading to an increase in packaging complexity, size and cost, as illustrated in FIG. 3. FIG. 3 presents a conventional optical layout of an HDRI Rx employing a reflective actuator 303 working on a collimated beam. Compared to the optical layout shown in FIG. 2, the optical layout in FIG. 3 employs an additional folding mirror 304 and a focusing lens 305, thereby increasing the component count. The output from an input fiber 301 propagates through a collimating lens 302, is reflected by the VOA actuator 303 and the folding mirror 304, propagates through the focusing lens 305, and is coupled to the photodetector 306. The folding mirror 304 introduces lateral placement offset of the photodetector 306, introducing asymmetry to the package and increasing its size. The focusing lens 305 contributes to an increase in packaging size and alignment complexity. The surfaces of the reflective VOA actuator 303 and the folding mirror 304 are oriented at an angle to the plane of the photodiode 306, adding to the packaging complexity. Integration of a reflective MEMS structure (as the VOA actuator 303) into the HDRI Rx comes with increased packaging size, complexity and cost.
There is a clear trade-off between the packaging complexity of the integrated receiver and the choice of the MEMS actuator used to achieve attenuation, i.e. a transmissive beam blocker or a reflective one.
An object of the present invention is to overcome the shortcomings of the prior art by providing a small sized and inexpensive HDRI Rx packaging solution with reduced complexity employing reflective MEMS VOA structures.